In order to change the inclination in which a borehole is being drilled, it has been a common practice to place in the downhole assembly a so-called "bent sub" which is a special piece of pipe that is made with a small bend angle between upper and lower portions thereof. The bent sub usually is connected to the top of the downhole motor which drives the bit in response to circulation of drilling fluids, or it can be a part of the assembly between the motor and its bearing section. The presence of a bend angle causes a gradual change in the inclination of the bottom portion of the borehole as the bit drills ahead, with the result that the borehole is formed along a curved path until a desired new inclination is achieved. The presence of a bend angle also allows torque that is applied to the drill string at the surface to be used to steer the bit to the right or to the left to achieve a change in the azimuth of the borehole. However, when the conventional type of bent sub is used, the drill string must be removed from the well in order to position the bent sub in, or take it out of, the downhole assembly.
The round trips of the drill pipe are necessary to insert and remove the typical bent sub are time consuming and very costly. There has been a long-felt need for a workable bent apparatus that is constructed in a manner such that a bend angle can be established in the tool downhole to enable a change in hole inclination to be accomplished when the need arises, and the bend angle eliminated downhole when straight-ahead drilling is desired. This need has become acute because of multiple well bores that are drilled from a single platform, and as horizontal drilling practices have come into common usage as a means for increasing the productivity of wells.
Bent sub assemblies that have an adjustable bend angle have been proposed, but are not considered to be commercially practical for various reasons. For example, devices such as those shown in U.S. Pat. Nos. 4,745,982, 4,813,497, 4,836,303, 4,220,214, 4,240,512, and 4,303,135 have to be removed from the well for adjustment of the bend angle, which necessitates a time consuming and expensive round trip of the drill string. Another adjustable bent sub which is described in U.S. Pat. No. 4,077,657, also must be hoisted up to the surface for adjustment. Proposals for downhole adjustable bent subs are shown in U.S. Pat. Nos. 4,655,299, 4,895,214 and 4,596,294, however these systems require surface manipulation of flow rates of the drilling fluids to achieve different downhole states of the bent sub. Since most wells, or sections thereof, are drilled using an optimum mud flow rate, particularly where a downhole motor is being used, changes in such flow rate are undesirable because they can affect the performance of the motor. Others downhole adjustable systems are illustrated in U.S. Pat. Nos. 4,884,643 and 4,374,547, however these devices include ratchets and continuous jay slots and pin arrangements which are mechanically complex and not particularly sturdy.
The general object of the present invention is to provide a new and improved bent sub assembly that can be adjusted downhole to control the bend angle without removing the drill pipe from the well.
Another object of the present invention is to provide a new and improved bent assembly that can be operated downhole to cause straight-ahead drilling, or adjusted to effect a change in the well bore inclination.
Another object of the present invention is to provide a new and improved bent housing apparatus that can be operated downhole so as to change its configuration from one where a lower portion thereof is coaxial with the longitudinal axis of the drilling motor for straight-ahead drilling, and another where such lower portion is at an angle with respect to such longitudinal axis for drilling a curved borehole.
Still another object of the present invention is to provide new and improved methods of controlling the bend angle of a downhole adjustable bent apparatus.